1. Field of the Invention
This invention relates generally to resistance bridge networks and more specifically to their use in electronic thermometers. The invention also relates to resistance thermometers having the properties of providing a fast and accurate indication of temperature, particularly the temperature of a human being. The invention includes a method for providing a sensible, fast, reliable, and low power indication of small changes in temperature. This is particularly obtained by providing a novel probe and a sheath which has the additional advantage of being disposable.
2. Description of the Prior Art
Electronic thermometers of the prior art have included temperature sensing devices the resistance of which have varied in a known manner with their temperatures. The resistance of the temperature sensing devices typically has been measured by a Wheatstone bridge. The characteristics of the null detector used in electronic temperatures have been particularly important since the variations of resistors with temperatures have been slight and the slight variations of resistances have provided only minute variations in the indicating current. Stable resistance thermometers could be made with complex and expensive null detectors but these would be impractical. As a substitute, simple and less expensive null detectors have been used at a great sacrifice in accuracy.
In addition, the less expensive but inaccurate null detectors have been highly consumptive of power. For example, where batteries have been used to power the thermometers, typically less than 50 temperatures have been taken on a single battery charge.
It can be appreciated that the accuracy of the Wheatstone bridge is highly dependent upon the ability of the null detector to sense very small currents. Indeed, the closer the resistance ratios are to equality, the smaller the magnitude of the current flowing through the null detector. The prior art has provided null detectors which can satisfactorily measure very small current but these detectors been exceptionally expensive. For this reason accurate Wheatstone bridges are typically found in ultra-precise metrology laboratories where the accurate measurement of resistance is the sole criteria. In more practical applications, the Wheatstone bridge has been cumbersome, relatively difficult to operate, and far too expensive.
The temperature sensing devices in the present electronic thermometers have also been unsatisfactory in providing a resistance which predictably varies with its temperature. Thermistors have been used but their resistance/temperature characteristics have varied with age. Compunding the problem is the fact that the change in resistance of a thermistor is a nonlinear function of temperature. For these reasons, the temperature sensing devices have been unreliable so that even if its resistance could be accurately measured, the measurement might not provide an accurate indication of the temperature.
The temperature sensing devices of the prior art have a relatively large mass the temperature of which must be changed in order to affect its resistance. It is well known, however, that a large mass requires more heat to change its temperatures than a smaller mass of the same material. In the case of an electronic thermometer this heat has typically been provided by the tissue of the patient whose temperature is being taken. As the tissue region gives up heat to the thermistor, the temperature of the region falls in proportion to the mass of the temperature sensing device. It follows that if the temperature of the tissue has fallen, the device cannot accurately reflect the original temperature of the region unless the region is reheated. Since the period of time required to reheat the region has also depended upon the mass of the temperature sensing device, it can be appreciated that the mass of the probe has had a great effect upon the length of time needed to take a temperature. Adding to this problem is the fact that heat is continually lost through the structure and wires supporting the temperature sensing device. Even when the tissue has acquired enough heat to raise its temperature, this continuing loss of heat to the structure indefinitely lowers the tissue temperature. To take a single temperature a period in excess of two minutes has been typical unless a high degree of operator's skill is relied upon in which case, a period of 15 seconds has been claimed.
The prior art has disclosed disposable sheaths which are designed to fit over the temperature sensing device on the end of a probe. These sheaths also have continuously drawn heat from the tissue. The sheaths have been constructed in a relatively complex and expensive manner, for example, in most cases they have been provided with a metal tip. To compound the problem, the metal tips have increased the mass of the region being heated.
For these reasons, the present electronic thermometers have been unsatisfactory in heating the thermistor and accurately developing a resistance which is predictable in view of the temperature of the thermistor. Furthermore, the relatively high current requirements of the bridges have substantially limited the portability of the electronic thermometers.